880 research outputs found
A trap-based pulsed positron beam optimised for positronium laser spectroscopy
We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 Ă 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cmâ3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 â 11 transition in electric fields
Multi-source self-calibration: Unveiling the microJy population of compact radio sources
Context. Very Long Baseline Interferometry (VLBI) data are extremely
sensitive to the phase stability of the VLBI array. This is especially
important when we reach {\mu}Jy r.m.s. sensitivities. Calibration using
standard phase referencing techniques is often used to improve the phase
stability of VLBI data but the results are often not optimal. This is evident
in blank fields that do not have in-beam calibrators. Aims. We present a
calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be
used after standard phase referencing on wide-field VLBI observations. This is
tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field-North and
the Hubble Flanking Fields. Methods. MSSC uses multiple target sources detected
in the field via standard phase referencing techniques and modifies the
visibili- ties so that each data set approximates to a point source. These are
combined to increase the signal to noise and permit self-calibration. In
principle, this should allow residual phase changes caused by the troposphere
and ionosphere to be corrected. By means of faceting, the technique can also be
used for direction dependent calibration. Results. Phase corrections, derived
using MSSC, were applied to a wide-field VLBI data set of the HDF-N comprising
of 699 phase centres. MSSC was found to perform considerably better than
standard phase referencing and single source self-calibration. All detected
sources exhibited dramatic improvements in dynamic range. Using MSSC, one
source reached the detection threshold taking the total detected sources to
twenty. 60% of these sources can now be imaged with uniform weighting compared
to just 45% with standard phase referencing. The Parseltongue code which
implements MSSC has been released and made publicly available to the
astronomical community (https://github.com/jradcliffe5/multi_self_cal).Comment: 7 pages, 4 figures, accepted to A&
Detection of fast radio transients with multiple stations: a case study using the Very Long Baseline Array
Recent investigations reveal an important new class of transient radio
phenomena that occur on sub-millisecond timescales. Often transient surveys'
data volumes are too large to archive exhaustively. Instead, an on-line
automatic system must excise impulsive interference and detect candidate events
in real-time. This work presents a case study using data from multiple
geographically distributed stations to perform simultaneous interference
excision and transient detection. We present several algorithms that
incorporate dedispersed data from multiple sites, and report experiments with a
commensal real-time transient detection system on the Very Long Baseline Array
(VLBA). We test the system using observations of pulsar B0329+54. The
multiple-station algorithms enhanced sensitivity for detection of individual
pulses. These strategies could improve detection performance for a future
generation of geographically distributed arrays such as the Australian Square
Kilometre Array Pathfinder and the Square Kilometre Array.Comment: 12 pages, 14 figures. Accepted for Ap
Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms
Predicted twenty years ago, positron binding to neutral atoms has not yet
been observed experimentally. A new scheme is proposed to detect positron-atom
bound states by colliding Rydberg positronium (Ps) with neutral atoms.
Estimates of the charge-transfer reaction cross section are obtained using the
first Born approximation for a selection of neutral atom targets and a wide
range of incident Ps energies and principal quantum numbers. We also estimate
the corresponding Ps ionization cross section. The accuracy of the calculations
is tested by comparison with earlier predictions for Ps charge transfer in
collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps
beam suitable for producing positron-atom bound states and estimate signal
rates based on the calculated cross sections and realistic experimental
parameters. We conclude that the proposed methodology is capable of producing
such states and of testing theoretical predictions of their binding energies.Comment: 19 pages, 11 figures, submitted to Phys. Rev.
Automatic Detection of Laryngeal Pathology on Sustained Vowels Using Short-Term Cepstral Parameters: Analysis of Performance and Theoretical Justification
The majority of speech signal analysis procedures for automatic detection of laryngeal pathologies mainly rely on parameters extracted from time domain processing. Moreover, calculation of these parameters often requires prior pitch period estimation; therefore, their validity heavily depends on the robustness of pitch detection. Within this paper, an alternative approach based on cepstral- domain processing is presented which has the advantage of not requiring pitch estimation, thus providing a gain in both simplicity and robustness. While the proposed scheme is similar to solutions based on Mel-frequency cepstral parameters, already present in literature, it has an easier physical interpretation while achieving similar performance standards
VLA Observations of Single Pulses from the Galactic Center Magnetar
We present the results of a 7-12 GHz phased-array study of the Galactic
center magnetar J1745-2900 with the Karl G. Jansky Very Large Array (VLA).
Using data from two 6.5 hour observations from September 2014, we find that the
average profile is comprised of several distinct components at these epochs and
is stable over day timescales and GHz frequencies. Comparison with
additional phased VLA data at 8.7 GHz shows significant profile changes on
longer timescales. The average profile at 7-12 GHz is dominated by the jitter
of relatively narrow pulses. The pulses in each of the four main profile
components seen in September 2014 are uncorrelated in phase and amplitude,
though there is a small but significant correlation in the occurrence of pulses
in two of the profile components. Using the brightest pulses, we measure the
dispersion and scattering parameters of J1745-2900. A joint fit of 38 pulses
gives a 10 GHz pulse broadening time of and a dispersion measure of . Both of these results are consistent with previous measurements,
which suggests that the scattering and dispersion measure of J1745-2900 may be
stable on timescales of several years.Comment: 20 pages, 10 figures, published in Ap
Nowhere to Hide: Radio-faint AGN in the GOODS-N field. I. Initial catalogue and radio properties
(Abridged) Conventional radio surveys of deep fields ordinarily have
arc-second scale resolutions often insufficient to reliably separate radio
emission in distant galaxies originating from star-formation and AGN-related
activity. Very long baseline interferometry (VLBI) can offer a solution by
identifying only the most compact radio emitting regions in galaxies at
cosmological distances where the high brightness temperatures (in excess of
K) can only be reliably attributed to AGN activity. We present the first
in a series of papers exploring the faint compact radio population using a new
wide-field VLBI survey of the GOODS-N field. The unparalleled sensitivity of
the European VLBI Network (EVN) will probe a luminosity range rarely seen in
deep wide-field VLBI observations, thus providing insights into the role of AGN
to radio luminosities of the order across cosmic
time. The newest VLBI techniques are used to completely cover an entire 7'.5
radius area to milliarcsecond resolutions, while bright radio sources ( mJy) are targeted up to 25 arcmin from the pointing centre. Multi-source
self-calibration, and a primary beam model for the EVN array are used to
correct for residual phase errors and primary beam attenuation respectively.
This paper presents the largest catalogue of VLBI detected sources in GOODS-N
comprising of 31 compact radio sources across a redshift range of 0.11-3.44,
almost three times more than previous VLBI surveys in this field. We provide a
machine-readable catalogue and introduce the radio properties of the detected
sources using complementary data from the e-MERLIN Galaxy Evolution survey
(eMERGE).Comment: 15 pages, 8 figures, accepted in A&A. Machine-readable table
available upon reques
Locating the intense interstellar scattering towards the inner Galaxy
We use VLBA+VLA observations to measure the sizes of the scatter-broadened
images of 6 of the most heavily scattered known pulsars: 3 within the Galactic
Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the
measured sizes with temporal pulse broadening data from the literature and
using the thin-screen approximation, we locate the scattering medium along the
line of sight to these 6 pulsars. At least two scattering screens are needed to
explain the observations of the GC sample. We show that the screen inferred by
previous observations of SGR J1745-2900 and Sgr A*, which must be located far
from the GC, falls off in strength on scales < 0.2 degree. A second scattering
component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC
produces most or all of the temporal broadening observed in the other GC
pulsars. Outside the GC, the scattering locations for all three pulsars are ~2
kpc from Earth, consistent with the distance of the Carina-Sagittarius or
Scutum spiral arm. For each object the 3D scattering origin coincides with a
known HII region (and in one case also a supernova remnant), suggesting that
such objects preferentially cause the intense interstellar scattering seen
towards the Galactic plane. We show that the HII regions should contribute >
25% of the total dispersion measure (DM) towards these pulsars, and calculate
reduced DM distances. Those distances for other pulsars lying behind HII
regions may be similarly overestimated.Comment: 16 pages, 10 figures, MNRAS, in pres
Microarcsecond VLBI pulsar astrometry with PSR II. parallax distances for 57 pulsars
We present the results of PSR, a large astrometric project targeting
radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric
database of 60 pulsars, we have obtained parallax-based distance measurements
for all but 3, with a parallax precision of typically 40 as and
approaching 10 as in the best cases. Our full sample doubles the number of
radio pulsars with a reliable (5) model-independent distance
constraint. Importantly, many of the newly measured pulsars are well outside
the solar neighbourhood, and so PSR brings a near-tenfold increase in the
number of pulsars with a reliable model-independent distance at kpc.
Using our sample along with previously published results, we show that even the
most recent models of the Galactic electron density distribution model contain
significant shortcomings, particularly at high Galactic latitudes. When
comparing our results to pulsar timing, two of the four millisecond pulsars in
our sample exhibit significant discrepancies in the estimates of proper motion
obtained by at least one pulsar timing array. With additional VLBI observations
to improve the absolute positional accuracy of our reference sources and an
expansion of the number of millisecond pulsars, we will be able to extend the
comparison of proper motion discrepancies to a larger sample of pulsar
reference positions, which will provide a much more sensitive test of the
applicability of the solar system ephemerides used for pulsar timing. Finally,
we use our large sample to estimate the typical accuracy attainable for
differential astrometry with the VLBA when observing pulsars, showing that for
sufficiently bright targets observed 8 times over 18 months, a parallax
uncertainty of 4 as per arcminute of separation between the pulsar and
calibrator can be expected.Comment: updated to version accepted by ApJ: 30 pages, 20 figures, 9 table
- âŚ